Motorized conveyors are known in the art. Most conveyors are well suited to longitudinally move objects from a first location to a second location along a predetermined path that may include horizontal potions, rising portions and descending portions.
It is often necessary to install torque limiting devices to motorized conveyors in order to restrict roller rotation when the required torque to rotate the roller is above a predetermined level. For example, if boxes are momentarily stopped on the predetermined path of the conveyor, for example, at a filling station, it is usually advantageous to stop the rotation of the rollers placed directly below the stopped boxes to reduce the wearing effect of the rollers on the bottom of these boxes. Furthermore, if a stream of boxes are stopped on the conveyor path, the sum of the pressures applied on the boxes by the push of the motorized rollers could cause the first boxes to be crushed. Of course, the rotation of the rollers should automatically resume when the boxes are no longer stopped.
U.S. Pat. No. 4,221,288 issued on Sep. 9, 1992 to George Rae discloses a motorized conveyor having rollers provided with tangential frictions torque limiting device. The term "tangential friction" is to be construed herein as a frictional force generated between two surfaces, usually curved, along a tangent generally perpendicular to a rotation axis, where the direction of the resulting friction force is opposite to the rotation direction. The torque limiting device described in the Rae patent includes three main elements: an adapter mounted to one end of the roller and having a predetermined external diameter, a driven element having an internal diameter slightly larger than the external diameter of the adapter and an endless belt exerting a pressure on the driven element to generate a tangential friction force between the internal surface of the driven element and the external surface of the adapter. The driven element is driven by a driving system of the motorized conveyor. When the force required to rotate the roller is less than the torque transmitted through the frictional engagement of the driven element and the adapter, the roller will be rotated. However, if the required torque to rotate the roller is greater than the torque transmitted through the frictional engagement, slippage will occur between the driven element and the adapter, therefore restricting the roller rotation.
U.S. Pat. No. 5,088,596 issued on Feb. 18, 1992 to Charles Agnoff discloses another motorized roller for a conveyor. The roller is provided with a motor installed inside the roller torque limiting device. In the present disclosure and in the appended claims, the term "lateral friction" is to be construed as a friction force created between two surfaces being generally perpendicular to a rotation axis; the direction of the friction force generated by lateral friction is opposite to the rotation direction. The torque limiting device described in this patent includes a clutch mechanism adapted to slip at a predetermined torque to protect the motor. The clutch mechanism includes a pair of disks frictionally engaged. The clutch means is associated with a drive member engaging the inner wall of the roller tube. When the force required to turn the roller tube is lower than the friction force present between the disks of the dutch mechanism, the roller tube is rotated. However, if the force required to rotate the roller tube is higher than this friction force, the dutch mechanism will slip and the rotation of the roller tube will be stopped.
The torque limiting devices disclosed by Rae and Agnoff include a common drawback: it is difficult to cool these torque limiting devices. Indeed, in the device described by Rae, the endless belt decreases the air contact area. On the other hand, in the conveyor roller described by Agnoff, the torque limiting device is mounted inside the roller which prevents air contact altogether.
European Patent Application No. 0,199,045 filed on Oct. 3, 1986 and naming Marcello Moriconi as the inventor and U.S. Pat. No. 4,469,220 issued to Becker on Sep. 4, 1984 both describe magnetic coupling between a drive sprocket and a conveyor roller. In these systems, the magnetic field produced by magnets is used to provide a torque limited connection between a drive sprocket and a roller. A small air gap is provided between the magnets and the roller.
Other types of mechanisms exist to limit the torque transmitted to a conveyor roller. For example, some rollers are filled with a viscous liquid and include internal projections or blades that must overcome the resistance of this liquid when the roller rotates. A drawback of this type torque limiting device is that the limit of the torque is dependant on the load imposed on the roller.